1. Field of the Invention
The invention relates to blind fasteners; and, more particularly, to a blind fastener having improved locking means.
2. Description of the Prior Art
Blind fasteners are known of a type having three pieces, a rivet sleeve, a headed core bolt and an expansible sleeve. The core bolt has a threaded shank with a breakneck portion separating the shank from a breakoff portion. The rivet sleeve has a head at one end to provide axial restraint on the driving side of two or more sheets and a tapered nose at its other end which extends beyond the blind side of the sheets. The sleeve is interposed between the nut's nose and the core bolt head. The fastener is set by rotating the core bolt with a driving tool engaged with the breakoff portion while holding the rivet sleeve in the sheets. The core bolt head is drawn inwardly towards the blind side of the sheets to expand the sleeve over the conical nose of the rivet sleeve to force the sleeve into compressive contact with the sheets. Upon reaching a predetermined torsional stress, the breakoff or driving portion of the core bolt severs from the shank at the breakneck groove. Axial restraint is provided in the set fastener by the expanded sleeve and the head of the rivet sleeve. This type of fastener is described in U.S. Pat. No. 2,765,699 to J. LaTorre.
Other fasteners known in the art, as the fasteners discussed in U.S. Pat. No. 3,643,544 to Massa and U.S. Pat. No. 2,531,270 to Hood, have a stop on the core bolt which abuts against the expansible sleeve and deform the sleeve in installing the fastener in a desired aperture. Certain prior art fasteners also have a locking sleeve or ring on the core bolt which enters the head of the rivet sleeve and serves to firmly lock the fastener in its installation hole, as for example, the ring 13 described and illustrated in U.S. Pat. No. 3,148,578. However, all of these fasteners have been found to be unsatisfactory in certain installations for one reason or another. These prior art fasteners require a close interaction between all the components so that the combination of friction, geometry and break-off diameter of the stem of the core bolt, during installation, is ideal. If tolerances are too excessive or metallurgical properties differ, or lubricants that are used are not consistent, these variable contribute to an inconsistent installation when the installation holes or grip thickness varies.
Most fasteners, as those in the aforementioned prior art, must break off at the same location regardless of grip condition. The fastener should expand within the installation hole so as to provide proper prestress of the drilled hole for resistance to fatigue or vibratory failures. This is accomplished by pulling the stem of the core bolt through the expansion sleeve where the stem diameter is larger than the inner diameter of the sleeve. As this stem pulls through, expansion of the sleeve is sufficient to provide the necessary installation hole-fill and the stem "necks down" or "wire draws" until the stem breaks off leaving the fastener in the fully installed condition (FIG. 4 of Hood, for example).
There is a need for a fastener which assures that it will install properly even under the worst conditions and that the stem never breaks prematurely. Much time is lost in the necessity of removing improperly installed rivets or fasteners. The fastener should be locked in its installation hole without the possibility of it becoming loose during use.